The invention relates to a method and an apparatus for the operation of a boiler installation with stoker-like firing, comprising a stoker chamber with a burner grating for the supply of the primary air, nozzles distributed over the periphery and a tubular boiler placed coaxially adjacent to the stoker chamber with an approximately circular inner chamber and annular gas aspiration.
Boiler installations with stoker-like firing are known in the prior art. In them the primary air is urged under pressure through the bars of the combustion grate which defines a bowl-shaped bottom for the fuel. The combustion takes place in two phases with a pre-combustion taking place in the bowl defined by the grating bars, while a post-combustion occurs, also called residual combustion, in the stoker chamber located above the bowl. The fuels burned in such installations develop, under a combustion of this kind, gases burnable on a large scale to which secondary air is admixed through nozzles placed in a crown-like arrangement, whereby the combustion in the stoker chamber takes place substantially free from residue.
It is known to design the firing equipment with a vertical axis and with an approximately circular cross-section, and to place the tubular boiler immediately above the stoker chamber which is open on top, so that the inner pipe crown practically forms a continuation of the stoker chamber. The inner chamber accommodating the pipe coils or pipe clusters is closed to a large extent at the top side, and the combustion gases are aspirated after having transferred their heat annularly to the pipe clusters, whereby it is possible to guide the gases several times annularly and, under reversal in axial direction, through pipe clusters surrounding each other concentrically.
In such boiler installations one considerable problem consists in that very high temperatures build up under optimal operation in the stoker chamber, so that the chamber walls are exposed to high thermal loads. It has been demonstrated in the art that a reduction of this thermal load, so that an optimal and economical operation is possible without too frequent shutdowns of the stoker firing system for a repair of the chamber walls, is extremely difficult. Therefore, the installations in most cases must be designed and built with higher expenses than what is necessary for actual operations under optimal conditions. This serious problem adversely affected the extended application of such boiler installations, although in principle they are extremely simple in their design and very economical in their operation, particularly since it became possible to provide for a reliable and satisfactory removal of the ashes without the necessity of a shutdown of the installation.
The invention is based on the problem of disclosing a method and an apparatus of the type defined initially in detail, with the aid of which it is possible to operate such a boiler installation with stoker firing under optimal conditions from a combustion-engineering point of view, without the need of frequent shutdowns for repairs at the chamber wall means. In this respect, care shall be taken primarily that with an improved manner of operation the installation can be built in a more space-saving manner and at a lower cost.
According to the invention this problem is solved by supplying the stoker chamber with primary air and secondary air sufficient only for a partial combustion of the fuel, by substantially constricting the cross-section of the flow of combustion gases and inflammable gases discharging from the stoker chamber, and by generating a pressure reduced with respect to the ambient pressure, whereby tertiary air is introduced tangentially at the circumference of the constricted flow at a pressure increased with regard to the ambient pressure and, subsequently, the cross-section of the entire flow again is flared upon its entry into the interior chamber.
As a result of the incomplete combustion in the grate bowl and in the stoker chamber, it is possible to maintain the temperature in the stoker chamber markedly lower than used before, so that the thermal load to which the chamber walls are exposed is substantially less. This makes it possible to construct the firing stoker with smaller dimensions and with less expense, whereby the lining materials of the stoker chamber have a substantially longer life than in the firing systems according to the prior art. The after-burning of the combustible gases produced in the stoker chamber takes place partly in the interior chamber of the tubular boiler. The air sufficient for this post combustion is supplied only at the area of connection between the stoker chamber and the interior chamber and, in fact, is supplied in a special manner. The feeding takes place under pressure and in a tangential direction in such a manner that a rotating air current results in the constriction or connection area between the stoker chamber and the boiler area, said rotation taking place along the inner wall means of the connection area. This current further constricts the gas flow and also converts it gradually into a current which rotates or ascends spirally and upwardly, and, based on the centrifugal forces behind the constriction, it has a tendency of ascending in close adherence at the pipe clusters forming the inner wall means of the interior chamber. This results in an adequate blending of air and gas mixture, so that the post combustion takes place preponderantly in the inner chamber. As a result of these described measures, such a strong subpressure is generated in the constriction area as compared with the ambient pressure existing at the same time that, in spite of the supply of tertiary air in this area, part of the gas located in the interior chamber of the boiler forms a core-like flow which reverses with spiral-like tendency at the ceiling of the boiler area and is oriented toward the point of constriction. This assures a complete post combustion of all flammable gases in the gas mixture, and assures an optimal thermal utilization of the gases. As a result of the return flow in the center of the combustion chamber, torn off unburned fuel particles carried along in the flow are returned to the fuel bed.
The extent of the constriction, the pressure, and the quantity of supplied tertiary air depend, among other factors, on the kind and composition of the fuel as well as on the extent of the incomplete combustion in the stoker chamber. It was found to be expedient to constrict the gas flow to about 1/3 to 1/4 of the cross-sectional area of the stoker chamber, and then to expand it to at least again approximately the cross-sectional area of the stoker chamber.
The invention is particularly appropriate with the installation arranged on a vertical axis, where the stoker chamber and the tubular boiler are immediately superposed and have an approximately circular cross-section.
For the practice of the method, the invention thus contemplates an apparatus where the stoker chamber is connected with the interior chamber of the boiler via a nozzle-like installation, and where in the connection area an installation is provided for the tangential supply of tertiary air. The nozzle-like constriction may be considered as a kind of Venturi nozzle. The mechanism for the supply of tertiary air appropriately is designed with an annular shape, and represents, at the same time, the most narrow point of the nozzle-like constriction. The super pressure at which the tertiary air is supplied is so adjusted that a rotating annular current develops at the narrowest location, which is so stable that it is not substantially destroyed by the gas mixture which ascends with increasing velocity. The rotational speed of this current will be adequate for expanding the total current after passage of the current through the most narrow point of the constriction in a spiral fashion, and in close contact with the pipe clusters in cross-section. The high gas velocity in the inner chamber of the boiler causes a better heat transfer coefficient between the combustion gases and the boiler wall.
The extent of the incomplete combustion in the stoker chamber can be controlled accurately by arranging the nozzles with a crown-like distribution over the circumferential wall of the stoker chamber, for the supply of secondary air opposite the stoker chamber axis, with a tilt which varies from the exterior to the interior and from the bottom down with respect to the axis of the installation. The differential slope may be in a range from about 10.degree. to 40.degree.. It preferably amounts to about 15.degree. and 35.degree., and the nozzles of different slope are preferably alternated in circumferential direction. In that manner, the secondary air can be so distributed in the stoker chamber over the surface of the material to be burned that an accurate and uniform control of the combustion and gasification is possible in this area.